The manufacture of components by solid state powder metallurgy (PM) occurs by pressing a powder of a metal, a metal alloy, or a metal composite in a die, which is followed by solid state sintering at a temperature below the solidus temperature of the powder metal. Component parts in the automotive industry are often made by solid state powder metallurgical methods.
Sintering below solidus temperature has certain advantages. For example, the compressed powder undergoes relatively little shrinkage. Furthermore, the compressed powder undergoes relatively little shape change during sintering. The shrinkage is generally less than 2 vol. %, which compares favorably to liquid phase sintering, in which the material may shrink by up to 15 to 20 vol. %. Accordingly, shape retention of solid state sintering is much better than that of liquid phase sintering. Moreover, since solid state powder metallurgy occurs below the solidus temperature, it is more cost effective than liquid state sintering, which is typically done at a temperature above solidus but below the liquidus temperature.
Components formed by solid state sintering, however, may not be fully dense and are often characterized by a high porosity level of 10 to 20 vol. %. Although the component parts have enough bulk strength for the intended applications, the surface is often not resistant to wear and corrosion because the alloys selected are typically ferrous alloys, and the porous surface allows penetration of air and water. In order to enhance surface wear and corrosion resistance, the industry has adopted a variety of surface treatments, such as carburizing, nitriding, steam treating, burnishing, and induction hardening.